The "c" in 25c is a whacky shorthand reference to the "c" in "700c," which was a classic alphabetical scheme to distinguish 700c from sizes like 700b or 700d.

Descriptions like "25c" are technically ambiguous, and only manage to be coherent because other "c"-labeled sizes (like 650c) are less popular than 700c.

The "c" never meant millimeters. Nor would it make sense as shorthand if it did, since then you'd still have to reference the wheel diameter in some way so that people know what you want: "I'd like a 700c x 25c tire."